What type of electromagnetic radiation is used to keep food warm?

Microwave ovens are often used to cook the food in the first place, but that's not what I think this question is referring to.

In some restaurants, you'll often see a certain shelf between the kitchen and the dining area, where the cooks put the plates that are waiting to be picked up by the waiters. There are usually a row of lights over this shelf, that shine on the plates to keep the food warm. Those are " infrared " lights. When I was growing up, they were also called "heat lamps".

The thing about them is this ... if you want to look at it this way: They're light bulbs that are designed and built to have extra-LOW efficiency. Of the total electrical energy that one of these bulbs uses, only a SMALL amount of it becomes light energy, and a LARGE amount of it becomes heat energy.